How to get to the primary key of a self tracking entity?

Question

I am trying to create a generic method that will retrieve an item by its id:

public T GetByID(int id)
{
    return (T) context.GetObjectByKey(
        new System.Data.EntityKey(context.DefaultContainerName + "." +
             context.CreateObjectSet<T>().EntitySet.Name, 
             "ProductID", id));
}

Basically I am able to infer the entity name from T, however I have no idea how to figure out what the primary key is for an entity?

Answer 1

Here's a snippet I have.. I hope it helps. Now that I look at it, I think I could improve it as well.

    _qualifiedTypeName = _context.DefaultContainerName + "." + _baseTypeName;
    Type baseType = GetBaseType(typeof(T));
    _baseTypeName = baseType.Name.ToString();

    PropertyInfo[] entityProperties = baseType.GetProperties();
    List<string> keyList = new List<string>();

    foreach (PropertyInfo prop in entityProperties) 
    {
      object[] attrs = prop.GetCustomAttributes(false);
      foreach (object obj in attrs)                
      {
        if (obj.GetType() == typeof(EdmScalarPropertyAttribute))
        {
          EdmScalarPropertyAttribute attr = (EdmScalarPropertyAttribute)obj;
          if (attr.EntityKeyProperty) keyList.Add(prop.Name);
         }
       }
    }
    if (keyList.Count > 0)
    {
      _keyName = keyList[0];
    }

and then you'd return it like:

EntityKey key = new EntityKey(_qualifiedTypeName, _keyName, id);
return (T)_context.GetObjectByKey(key);

and it looks like I got the above from here: http://blog.vascooliveira.com/how-to-check-for-an-entity-objects-entitykey-properties/

Answer 2

I ended up creating my own attribute and modifying the T4 template to place that attribute above the primary key column. Here are the steps I took:

1. Add the following above the [DataMember] attribute in the T4 template:
   <#if (ef.IsKey(edmProperty)) {#> [PrimaryKeyAttribute] <#}#>
2. Create the PrimaryKeyAttribute:

    [AttributeUsage(AttributeTargets.Property)]
    public class PrimaryKeyAttribute : Attribute
    {}

1. Introduce a helper method to determine the primary key of an entity:

        private string GetPrimaryKey()
        {
            string primaryKey = string.Empty;

            PropertyInfo[] entityProperties = typeof(K).GetProperties();

            foreach (PropertyInfo prop in entityProperties)
            {
                object[] attrs = prop.GetCustomAttributes(false);
                foreach (object obj in attrs)
                {
                    if (obj.GetType() == typeof(PrimaryKeyAttribute))
                    {
                        primaryKey = prop.Name;
                        break;
                    }
                }
            }

            if (string.IsNullOrEmpty(primaryKey))
                throw new Exception("Cannot determine entity's primary key");

            return primaryKey;
        }

1. Finally write the generic GetByID as such:

        public T GetByID(int id)
        {
            return (T)context.GetObjectByKey(new EntityKey(context.DefaultContainerName 
                                                + "." + context.CreateObjectSet().EntitySet.Name
                                                , GetPrimaryKey(), id));            
        }